I figured this out, after an hour of head-scratching. Recording it here, in case others have the same problem.
Initial issue was that I didn't like the default color scheme, and wanted to change things.
1 -- make a new scheme, via edit -> preferences -> color scheme
2 -- use the new scheme, via edit -> preferences -> appearance
Supposedly you can change the existing scheme in-place, but that didn't work for me. An alternative is to skip step 1, and switch to an already-created scheme. If you were happy with the way things were once upon a time, you can switch back by selecting the 'Classic' scheme.
(And, yes, given the "didn't work for me" issue, I don't know how you'd experiment, except by tediously applying the above steps over and over, until perhaps 'MyScheme23' is something you're happy with.)